What's an easier green power source to acquire than sun or wind? Random ambient vibrations, that's what. And that's exactly how some new generators juice up.

The devices, created by researchers at the University of Michigan, aren't going to be powering your cars anytime soon, but they'll be able to gather enough energy to power watches, pacemaker or wireless sensors.

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The vibrations they pick up are from things like traffic on bridges, machinery operating in factories or you swinging your arms around.

The researchers have built three prototypes and a fourth is forthcoming. In two of the generators, the energy conversion is performed through electromagnetic induction, in which a coil is subjected to a varying magnetic field. This is a process similar to how large-scale generators in big power plants operate.

The latest and smallest device, which measures one cubic centimeter, uses a piezoelectric material, which is a type of material that produces charge when it is stressed. This version has applications in infrastructure health monitoring. The generators could one day power bridge sensors that would warn inspectors of cracks or corrosion before human eyes could discern problems.